Period (periodic table)

In the periodic table of the elements, elements are arranged in a series of rows (or periods) so that those with similar properties appear in a column. Elements of the same period have the same number of electron shells; with each group across a period, the elements have one more proton and electron and become less metallic. This arrangement reflects the periodic recurrence of similar properties as the atomic number increases. For example, the alkaline metals lie in one group (group 1) and share similar properties, such as high reactivity and the tendency to lose one electron to arrive at a noble-gas electronic configuration. The periodic table of elements has a total of 118 elements.

The Madelung energy ordering rule describes the order in which orbitals are arranged by increasing energy according to the Madelung rule. Each diagonal corresponds to a different value of n + l.

Modern quantum mechanics explains these periodic trends in properties in terms of electron shells. As atomic number increases, shells fill with electrons in approximately the order shown at right. The filling of each shell corresponds to a row in the table.

In the s-block and p-block of the periodic table, elements within the same period generally do not exhibit trends and similarities in properties (vertical trends down groups are more significant). However in the d-block, trends across periods become significant, and in the f-block elements show a high degree of similarity across periods.

The first period contains fewer elements than any other, with only two, hydrogen and helium. They therefore do not follow the octet rule. Chemically, helium behaves as a noble gas, and thus is taken to be part of the group 18 elements. However, in terms of its nuclear structure it belongs to the s block, and is therefore sometimes classified as a group 2 element, or simultaneously both 2 and 18. Hydrogen readily loses and gains an electron, and so behaves chemically as both a group 1 and a group 17 element.

Helium (He) exists only as a gas except in extreme conditions.[3] It is the second lightest element and is the second most abundant in the universe.[4] Most helium was formed during the Big Bang, but new helium is created through nuclear fusion of hydrogen in stars.[5] On Earth, helium is relatively rare, only occurring as a byproduct of the natural decay of some radioactive elements.[6] Such 'radiogenic' helium is trapped within natural gas in concentrations of up to seven percent by volume.[7]

Period 2 elements involve the 2s and 2porbitals. They include the biologically most essential elements besides hydrogen: carbon, nitrogen, and oxygen.

Lithium (Li) is the lightest metal and the least dense solid element.[8] In its non-ionized state it is one of the most reactive elements, and so is only ever found naturally in compounds. It is the heaviest primordial element forged in large quantities during the Big Bang.

Boron (B) does not occur naturally as a free element, but in compounds such as borates. It is an essential plant micronutrient, required for cell wall strength and development, cell division, seed and fruit development, sugar transport and hormone development,[11][12] though high levels are toxic.

Carbon (C) is the fourth most abundant element in the universe by mass after hydrogen, helium and oxygen[13] and is the second most abundant element in the human body by mass after oxygen,[14] the third most abundant by number of atoms.[15] There are an almost infinite number of compounds that contain carbon due to carbon's ability to form long stable chains of C—C bonds.[16][17] All organic compounds, those essential for life, contain at least one atom of carbon;[16][17] combined with hydrogen, oxygen, nitrogen, sulfur, and phosphorus, carbon is the basis of every important biological compound.[17]

Nitrogen (N) is found mainly as mostly inertdiatomic gas, N2, which makes up 78% of the Earth's atmosphere. It is an essential component of proteins and therefore of life.

Oxygen (O) comprising 21% of the atmosphere and is required for respiration by all (or nearly all) animals, as well as being the principal component of water. Oxygen is the third most abundant element in the universe, and oxygen compounds dominate the Earth's crust.

Fluorine (F) is the most reactive element in its non-ionized state, and so is never found that way in nature.

All elements of period 7 are radioactive. This period contains the heaviest element which occurs naturally on earth, californium. All of the subsequent elements in the period have been synthesized artificially. Whilst one of these (einsteinium) is now available in macroscopic quantities, most are extremely rare, having only been prepared in microgram amounts or less. Some of the later elements have only ever been identified in laboratories in quantities of a few atoms at a time.

Although the rarity of many of these elements means that experimental results are not very extensive, periodic and group trends in behaviour appear to be less well defined for period 7 than for other periods. Whilst francium and radium do show typical properties of Groups 1 and 2 respectively, the actinides display a much greater variety of behaviour and oxidation states than the lanthanides. These peculiarities of period 7 may be due to a variety of factors, including a large degree of spin-orbit coupling and relativistic effects, ultimately caused by the very high positive electrical charge from their massive atomic nuclei.

No element of the eighth period has yet been synthesized. A g-block is predicted. It is not clear if all elements predicted for the eighth period are in fact physically possible. There may therefore be no ninth period.